1. Technical Field
This invention relates to a method for producing a semiconductor substrate, and more particularly to a method for producing a semiconductor substrate through a lamination process of directly laminating silicon wafers to each other without using an oxide film.
2. Related Art
Recently, semiconductor substrates having such a SOI structure that a silicon layer or a so-called SOI layer is formed on an oxide film are applied as a wafer for high-performance LSI in electron devices because they are adaptable for the speeding-up of the device and are low in the consumption power and excellent in the pressure resistance, environment resistance and the like. Particularly, it is demanded to produce SOI wafer having a higher quality in association with a higher integration of semiconductor device, and hence it is increasingly requested to form a laminated wafer by thinning a buried oxide film, for example, up to a thickness of about 20 nm as compared with the conventional ones or by directly laminating silicon wafers to each other without using the oxide film.
As to the semiconductor substrate of SOI structure, there are known a so-called SIMOX method wherein oxygen ions are implanted into a silicon wafer at a high concentration and then subjected to a heat treatment at a higher temperature to form an oxide film in its interior, and a method called as a lamination process. In this lamination process, an oxide film is formed on at least one of a wafer for an active layer forming SOI layer and a wafer for a support substrate turning a support substrate and the wafer for the active layer is laminated onto the wafer for the substrate through the oxide film and thereafter the wafer for the active layer is thinned to produce a semiconductor substrate wherein SOI layer is formed on the buried oxide film as an insulating film.
Further, the lamination process can be classified into a grinding and polishing process, a PACE (plasma assisted chemical etching) process, an ion implantation exfoliation process (which is also called as Smart Cut (registered trademark) process), an ELTRAN process and the like. Among them, the ion implantation exfoliation process is frequently used in view of advantageous points that the crystallinity of the active layer is good, the uniformity of the thickness of the active layer is good, the flatness of the surface is good and the like.
The production procedure of the semiconductor substrate through the ion implantation exfoliation process is shown in FIG. 1 with respect to a case that silicon wafers are directly laminated to each other without using an oxide film. That is, a wafer 1 for an active layer and a wafer 2 for a support substrate are previously provided (step (a)), and hydrogen ions (or inert gas ions) are implanted into at least one of these wafers (the wafer 1 for the active layer in the illustrated embodiment) to form an ion implanted layer 3 in the interior of the wafer 1 for the active layer (step (b)). Next, the ion implanted face of the wafer 1 for the active layer is laminated onto the wafer 2 for the support substrate (step (c)) and thereafter subjected to an exfoliation heat treatment to partly exfoliate the wafer 1 for the active layer using the ion implanted layer 3 as a cleavage plane (exfoliation face) (step (d)), and thereafter an oxidation treatment is again conducted for removing a damaged layer formed on the surface of the active layer (step (e)), and a step (f) of removing the resulting oxide film 4 is conducted and a planarization treatment is conducted to produce a semiconductor substrate 6 in which a silicon layer 5 is formed on the wafer 2 for the support substrate (step (g)).
In the production of the laminated wafer through the ion implantation exfoliation process, the preparation of the wafer without forming the oxide film is conducted by laminating the wafer for the active layer and the wafer for the support substrate to each other without forming the oxide film on both the wafers.
However, when the wafer is prepared without forming the buried oxide film, if the exfoliation heat treatment is carried out after the lamination of the wafers, blisters or voids are generated between the wafer for the support substrate and the wafer for the active layer.
That is, when the semiconductor substrate is prepared by the conventional lamination process, defects such as voids, blisters may be generated on the laminated interface. These void or blister defects tend to be frequently generated as the thickness of the buried oxide film existing between the two semiconductor substrates becomes thinner and particularly come into a serious problem in the production of the laminated semiconductor wafer having no oxide film.
Since the voids or blisters are frequently generated as the thickness of the buried oxide film existing between the two semiconductor wafers become thin, JP-A-2004-259970 proposes a countermeasure that the thickness of the wafer for the active layer is increased to increase the thickness of the active layer and raise the hardness of the active layer.
However, as the film thinning is proceeding with respect to the active layer, the thickening of the thickness of the active layer for raising the hardness at a midway step requires labor in the subsequent work for the film thinning and results in the deterioration of the quality. That is, when the thickness of the active layer is thick at the midway step, in order to obtain a final thickness of the active layer, it is required to reduce the thickness by thermal oxidation and removal of oxide film or by grinding or polishing work. As the working quantity (oxidation quantity, etching quantity, grinding or polishing quantity) increases, it is difficult to make the thickness of the active layer uniform.